Currently, information is displayed on sheets using permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information. Magnetically written data, however, is not visible.
World patent application PCT/WO 97/04398, entitled “Electronic Book With Multiple Display Pages”, is a thorough recitation of the art of thin, electronically written sheet display technologies. Disclosed is the assembling of multiple display sheets that are bound into a “book”, each sheet provided with means to individually address each page. The patent recites prior art in forming thin, electronically written pages, including flexible sheets; image modulating material formed from a bi-stable liquid crystal system, and thin metallic conductor lines on each page.
An early patent, U.S. Pat. No. 3,578,844, discloses a light modulating structure suitable for a display device. In the patent, cholesteric liquid crystal material is encapsulated by light penetrable gelatin and gum arabic capsules that are coated on a screen. The capsules were formed by emulsifying the cholesteric material in a gelatin solution using a blender to form droplets between 10 and 30 microns in diameter. The pH of the emulsion was changed to precipitate a gelatin coating over each droplet of cholesteric material. The gelatin was hardened and the capsules sieved from the solution. The capsules are then coated over a field-carrying surface to provide an electrically switchable image.
U.S. Pat. No. 3,600,060 to Churchill et al. discloses another process for providing cholesteric liquid crystals in a polymer matrix. The patent discloses emulsifying droplets of liquid crystal in a solution having a dissolved film-forming polymer. The patent further discloses coatings or films having droplets of cholesteric liquid crystal material between 1 and 50 microns in diameter. Suitable binders mentioned in the paper include gelatin, gum arabic, and other water-soluble polymers. Churchill et al. disclose the emulsion can be coated on a substrate, e.g., by means of a draw down applicator to a wet thickness of about 10 mils and air dried at about 25° C. Churchill et al. state that the layers can be dried to touch. In Example 6, 60 grams of cholesteric liquid crystal material is disposed in 100 cubic centimeters of an aqueous polymer solution, polyvinyl alcohol or gelatin, and heated in a Waring® blender to 70° C. by a heating jacket to form a desired emulsion, after which the emulsions were coated onto glass previously coated with tin oxide. No hardening agent is used by Churchill et al. Such coatings would be sensitive to environmental conditions.
U.S. Pat. No. 5,168,380 discloses the generation of spherical encapsulated nematic liquid crystal using a hardening agent to strengthen the containment medium. The nematic liquid crystal and dye mixture must be encapsulated to separate different chemistries in a multi-color display. A first material, a maleic anhydride derived copolymer is in solution with the liquid crystal, while a second material, polyvinyl alcohol is in solution with the water bath holding liquid-crystal domains. Incubation for one hour allowed a cross-linking reaction to occur at the interface boundary to form a robust encapsulating wall. Capsules are formed with different color nematic liquid-crystal-and-dye mixtures that are coated to form multi-color displays. The nematic materials do not retain an image. The process requires a significant incubation time to form the walls. It would be advantageous to create polymer-dispersed liquid-crystal domains without the encapsulation step.
Another technique for providing liquid crystal domains in a coating is disclosed in U.S. Pat. No. 4,673,255. A resin polymer is dissolved into a liquid crystal. The resulting solution is induced into a cavity between two conductors. The resin polymer phase separated from the liquid crystal to form microdroplets of the liquid crystal in a polymeric matrix. The phase separation can be thermally induced, solvent induced or polymerization induced to create domains of liquid crystal. These processes, however, require lengthy periods of time to polymerize and phase separate the polymer binder. Organic solvents used in such systems are of environmental concern and are sensitive to processing conditions.
In view of the above, the prior art discloses cholesteric material encapsulated using pH changes to precipitate gelatin capsule walls around cholesteric droplets. No hardening agents are used in the process. The prior art also discloses aqueous-coated nematic liquid crystals having a cross-linking agent. The resulting coatings do not have the property of memory. Phase separation systems for cholesterics disclose cross-linking agents in solution with cholesteric liquid crystal material. The cross-linking agent permits polymerization of the monomer to form a polymer-dispersed cholesteric layer. Such processes are slow.
The patents mentioning gelatin for use in a liquid-crystal display do not employ a gelatin hardener in a continuous binder matrix. Although hardened gelatin is used in photographs to harden the material, the need is not the same in liquid crystal displays in which the gelatin is typically protected by several layers of material including a plastic or glass substrate. Typically, liquid-crystal material is wicked between plates of glass. Furthermore, unless necessary, a gelatin hardener can be problematic for coating a gelatin material and may require more difficult manufacture.